Method of forming a trench capacitor

ABSTRACT

The present invention provides a method of forming trench capacitor with a sacrificial silicon nitride. A deep trench structure is formed in a substrate. A TEOS oxide layer is formed on the substrate and filled in said trench region, etched to a first level subsequently, wherein a portion of the TEOS oxide layer is remained in the trench region and a portion of the substrate exposed to form a trench sidewall. A thermally oxidation process is performed to form a collar oxide on the exposed substrate. A silicon nitride sidewall is formed on the collar oxide, then removing the residual TEOS oxide layer by wet etching. The trench region is then etched using the silicon nitride sidewall as a barrier to form a bottle shape trench region for increasing the surface of the trench region. A bottom cell plate is formed in the fresh trench region. The silicon nitride sidewall is removed. A dielectric film is formed along a surface of the bottom cell plate, the collar oxide, and the substrate, subsequently, a first conductive layer is formed on said dielectric film and refill in the trench region. The first conductive layer and the dielectric film are etched to expose a portion of the collar oxide, the exposed portion of the collar oxide is then etched by wet etching. A second conductive layer is formed on the first conductive layer and etched back to form a buried strap in the trench region.

FIELD OF THE INVENTION

The present invention relates to a trench capacitor, and more particularly, to a method of forming a bottle shape trench capacitor with a sacrificial silicon nitride sidewall to increase the surface of the capacitor.

BACKGROUND OF THE INVENTION

In recent years, the development of the semiconductor memory device has become highly integrated and having high packing density. The area occupied by a capacitor of a DRAM storage cell has shrunk, thus decreasing the capacitance of the capacitor owing to its smaller electrode surface area. However, a relatively large capacitance is required to achieve a high signal-to-noise ratio in reading the memory cell and to reduce soft errors (due to alpha particle interference). Therefore, it is desirable to reduce the cell dimensions and yet obtain a high capacitance, which achieves both high cell integration and reliable operation.

The capacitor type that is most typically used in DRAM memory cells is planar capacitor, which is relatively simple to manufacture. For very small memory cells, planar capacitor becomes very difficult to use reliably. One approach for increasing the capacitance while maintaining the high integration of the memory cells is directed toward the shape of the capacitor electrodes. In this approach, the polysilicon layer implementing the capacitor electrodes may have protrusions, cavities, fins, etc., to increasing the surface area of the capacitor electrode, so that increases its capacitance while maintaining the small area occupied on the substrate surface.

When DRAM cells are scaled down while maintaining the cell capacitance, three-dimensional cell structures, such as trench capacitors, are widely developed. One of advantages of the trench capacitor cell is its large capacitance and planar topography. One of the prior arts in accordance with the trench capacitor can be seen “A 0.6 μm² 256 Mb Trench DRAM Cell With Self-Aligned BuriEd STrap (BEST), L. Nesbit et al., 1993, IEEE, IEDM 93-627”. A unique feature of this cell is a self-aligned buried strap that forms at the intersection of the storage trench and the junction of the array device. However, it is difficult to make deep trenches with a high aspect ratio for high density DRAMs.

An example of a conventional deep trench capacitor is depicted in FIG. 1A to FIG. 1C. Referring to FIG. A, that shows a dielectric layer 102 is formed on a semiconductor substrate 101, and a trench region 100 is formed by dry etching down. After the trench is etched, the N-type capacitor plate 103 is formed by outdiffusing arsenic or phosphoric from the lower portion of the trench. An oxidized nitride (NO) storage node dielectric 104 is formed in the storage trench, followed by the deposition and controlled recess of a first trench fill of n+ polysilicon 105. See FIG. 1B, a collar oxide 106 is formed using conventional CVD process. The depth of buried strap is defined by a controlled etchback of a second polysilicon trench fill 107 and the removal of the exposed collar oxide. A third polysilicon film 108, which contacts the silicon substrate along the exposed trench sidewall, is deposited and recessed below the silicon substrate surface.

When the dimension of DRAM is close to deep-submicron, a new method for manufacturing small capacitors is needed. A method to fabricate a trench capacitor on a smaller occupancy space with higher capacitance is needed.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an improved method of forming a trench capacitor.

The other object of the present invention is that to provide a method of forming a trench capacitor with a sacrificial silicon nitride sidewall, which protects a collar oxide been etched or doped.

A first silicon nitride layer a thick oxide layer are formed on a silicon substrate, then pattern said first silicon nitride layer and said thick oxide layer to form a deep trench region in said substrate. Subsequently, said thick oxide layer is removed away by wet etching. A TEOS oxide layer is then formed on said first silicon nitride layer and filled in said trench region by conventional process. The TEOS oxide layer is etched to a first level by dry etching, wherein a portion of the TEOS oxide layer is remained in the trench region and a portion of the silicon substrate exposed in the trench region. A thermally oxidation process is performed to form a collar oxide on the exposed silicon substrate. A second silicon nitride layer is formed on the first silicon nitride layer, the collar oxide, and the residual TEOS oxide layer. The second silicon nitride layer is etched anisotropically to form a silicon nitride sidewall on said collar oxide, subsequently, removing the residual TEOS oxide layer by wet etching.

When the residual TEOS oxide has been etched, the trench region is then etched using the silicon nitride sidewall as a barrier. A freshly excavated trench is formed, wherein the surface of the fresh trench is increased. A dopant layer is formed in the fresh trench region for doping ions over the fresh trench region of lower the silicon nitride sidewall to form a bottom cell plate. In the other embodiment, the fresh trench is formed after the steps of doping for increasing the surface of the trench.

The silicon nitride sidewall is removed. A dielectric film is formed on the bottom cell plate, the collar oxide, and the first silicon nitride layer, subsequently, a first conductive layer is formed on said dielectric film. The first conductive layer and the dielectric film are etched to a second level to expose a portion of the collar oxide, the exposed portion of the collar oxide is then etched by wet etching. A second conductive layer is formed on the first conductive layer and etched back to form a buried strap in the trench region.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

FIG. 1A is a cross-sectional view of a semiconductor wafer illustrating the steps of forming a trench structure on a semiconductor substrate by conventional processes;

FIG. 1B is a cross-sectional view of a semiconductor wafer illustrating the steps of forming a collar oxide on the trench sidewall by conventional processes;

FIG. 1C is a cross-sectional view of a semiconductor wafer illustrating the steps of forming a buried strap in the trench structure by conventional processes;

FIG. 2 is a cross-sectional view of a semiconductor wafer illustrating the step of forming a deep trench region on the semiconductor substrate;

FIG. 3 is a cross-sectional view of a semiconductor wafer illustrating the step of forming a thermal collar oxide in the trench region;

FIG. 4 is a cross-sectional view of a semiconductor wafer illustrating the step of forming a sacrificial silicon nitride sidewall and a N-type bottom conductive plate in the trench region;

FIG. 5 is a cross-sectional view of a semiconductor wafer illustrating the step of forming a dielectric film and a conductive layer in the fresh trench region; and

FIG. 6 is a cross-sectional view of a semiconductor wafer illustrating the step of forming a buried strap in the trench region.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides an improved method for forming a trench capacitor with a sacrificial silicon nitride sidewall to protect a collar oxide from doping and etching. A trench capacitor constructed in accordance with the present invention is depicted in FIGS. 2-6.

Referring to FIG. 2, a single crystal P-type silicon substrate 201 with a <100> crystallographic orientation is used for the preferred embodiment. A silicon nitride layer 202, has a thickness about 2000 to 2500 Å, is formed on the silicon substrate 201 using a conventional low pressure chemical vapor deposition (LPCVD) or plasma enhanced chemical vapor deposition (PECVD). A thick oxide layer 203 with a thickness about 7000 to 8000 Å is then formed and patterned on the silicon nitride layer 202 to define a deep trench region 200, which is formed by dry etching down to a predeterminate depth into the P-type silicon substrate. The deep trench region has a depth about 7.0 to 8.0 μm and a width about 0.2 to 0.4 μm. In this embodiment, the etching process for forming the deep trench region 200 is preferably done by a standard plasma dry etch method or a reactive ion etch (RIE), which are conventional and well known, so that the processes are not particularly germane to the present invention. The thick oxide layer 203 is then removed away by wet etching.

Referring now to FIG. 3, a tetraethylorthosilicate (TEOS) oxide layer 204 is formed in the deep trench region 200 and on the silicon nitride layer 202. The TEOS oxide layer 204 is recess etched to a first level 20, which has a depth of about 1.3 to 1.6 μm. In this etching, a portion of silicon substrate 201 is exposed in the deep trench region 200 to form a trench sidewall. A thermally oxidation is performed to form a collar oxide 205 on the exposed silicon substrate 201. The reaction temperature of the thermally oxidation is about 900 to 1050° C. and the collar oxide has a width about 40 to 50 nm.

FIG. 4 shows that a thin silicon nitride layer is formed along a surface of the silicon nitride layer 202, the collar oxide 205 and the TEOS oxide layer 204. A silicon nitride sidewall 206 is formed using a spacer etching technique. The silicon nitride sidewall 206, has a width about 10 to 12 nm, prevents the covered areas from being oxidized, doped and etched. The TEOS oxide layer 204 is then removed by using a BOE (buffer oxide etching) solution or a diluted HF solution. The trench region is then etched for increasing the surface using the silicon nitride sidewall as a barrier. The process is done using a wet etching of HNO₃/CH₃COOH/HF/DI (deionized) H₂O solution or dry etching of HBr/Cl₂ to form a freshly excavated bottle-shaped trench region.

A N-type bottom plate 207 is formed in the P-type silicon substrate 201 by covering the freshly excavated trench with a thin layer of arsenic doped TEOS or phosphoric doped TEOS and allowing the N-type dopant to partially diffuse into the P-type silicon substrate by annealing. The doped TEOS layer and the silicon nitride sidewall 206 are then removed away by wet etching. The silicon nitride sidewall 206 is stripped away using hot phosphorus acid solution (H₃PO₄). Vapor-phase phosphorus doping also can form the N-type bottom plate, which is formed on the freshly excavated trench region.

Still referring to FIG. 4, in the other preferred embodiment, when the TEOS oxide layer 204 is removed, a N-type bottom plate 207 is formed in the P-type silicon substrate 201 by doping arsenic or phosphoric ion. A portion of the N-type bottom plate 207 is then etched to form a bottle-shaped trench region for increasing the surface using the silicon nitride sidewall as a barrier. The process of etching the N-type bottom plate 207 is done using a wet etching of HNO₃/CH₃COOH/HF/DI (deionized) H₂O solution or dry etching of HBr/Cl₂. The silicon nitride sidewall 206 is stripped away using hot phosphorus acid solution (H₃PO₄).

Next referring to FIG. 5, a dielectric film 208 of the trench capacitor is conformably deposited along the surface of the silicon nitride layer 202, the collar oxide 205 and the N-type bottom plate 207. Typically, the dielectric film 208 is preferably formed of either a double film of nitride/oxide (NO) film, a triple film of oxide/nitride/oxide (ONO), or any other high dielectric film such as tantalum oxide (Ta₂O₅), BST, PZT, PLZT. In the preferred embodiment, stacked nitride-oxide (NO) film is used as the material of the dielectric film 208. As is known in the art of DRAM capacitor fabrication, the NO film is reliable over silicon surface, and is typically used as a capacitor insulator. A first conductive layer 209 is deposited on the dielectric film 208 and refilled into the deep trench region 200 using a conventional LPCVD process. The first conductive layer 209 is preferably either doped polysilicon or in-situ doped polysilicon. The first conductive layer 209 is recess etched to the second level 21, which has a depth of about 280 to 310 nm. So that a portion of the dielectric film 208 is exposed and then etched by wet etching.

Referring to FIG. 6, after the exposed dielectric film is striped a portion of the collar oxide 205 is exposed. The exposed collar oxide is stripped away using conventional wet etching to lower than the second level, wherein the wet etching has a high susceptibility to the collar oxide. A second conductive layer 211 is deposited on the first conductive layer 209, and is then etched back to a third level for forming a buried strap 211. The third level is below the top surface of the silicon substrate. Thus the buried strap forms without using a separate mask. The second conductive layer is preferably either doped polysilicon or in-situ doped polysilicon.

The present invention provides a method of forming a trench capacitor with a sacrificial silicon nitride sidewall, in this work, we present a simple process to fabricate a self-aligned buried strap and a sacrificial silicon nitride sidewall to protect the collar oxide from wet etching. These technologies provide a trench capacitor with high capacitance and good performance.

As is understood by a person that skilled in the art, the foregoing preferred embodiment of the present invention are illustrated of the present invention rather than limiting of the present invention. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structure.

While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A method of forming a trench capacitor with a sacrificial silicon nitride sidewall, said method comprising the steps of: providing a silicon substrate; patterning said silicon substrate to form a trench region in said substrate; forming a silicon oxide layer on said silicon substrate and in said trench region; etching said silicon oxide layer to a first level, wherein a portion of said silicon oxide layer is remaining in said trench region, thereby a portion of a trench sidewall of said trench region is exposed; forming a collar oxide on said portion of said trench sidewall; forming a silicon nitride sidewall on said collar oxide; removing said portion of said silicon oxide layer in said trench region to expose a bottom portion of said trench region; etching said bottom portion of said trench region to form a fresh trench region, wherein said process is done by using said silicon nitride sidewall as a barrier; doping ions over said fresh trench region to form a bottom cell plate; removing said silicon nitride sidewall; forming a dielectric film along a surface of said bottom cell plate, said collar oxide, and said silicon substrate; forming a first conductive layer on said dielectric film and refilling in said trench region; etching said first conductive layer to a second level to form a storage node of said trench capacitor and exposing a portion of said collar oxide; etching said exposed dielectric film and portion of said collar oxide; forming a second conductive layer on said first conductive layer; and etching back said second conductive layer to a third level to form a buried strap in said trench region.
 2. The method according to claim 1, wherein said silicon oxide layer is formed of TEOS oxide.
 3. The method according to claim 1, wherein said collar oxide is formed by thermal oxidation.
 4. The method according to claim 1, wherein said step of etching exposed trench region is done by wet etching.
 5. The method according to claim 4, wherein said step of wet etching is done using HNO₃/CH₃COOH/HF/DI (deionized) H₂O solution.
 6. The method according to claim 1, wherein said step of etching exposed trench region is done by dry etching.
 7. The method according to claim 6, wherein said step of dry etching is done using HBr/Cl₂.
 8. The method according to claim 1, wherein said step of doping ions is performed by vapor-phase doping.
 9. The method according to claim 1, wherein said step of doping ions is performed by thermal diffusing.
 10. The method according to claim 1, wherein said doping ions is selected from the group of arsenic and phosphoric ions.
 11. The method according to claim 1, wherein said dielectric film is elected from the group of NO, ONO, Ta₂O₅, BST, PZT, and PLZT.
 12. The method according to claim 1, wherein said first conductive layer and said second conductive layer are selected from the group of doped polysilicon and in-situ doped polysilicon.
 13. The method according to claim 1, wherein said silicon nitride sidewall has a width about 10 to 12 nm.
 14. The method according to claim 1, wherein said third level is below a top surface of said silicon substrate.
 15. A method of forming a trench capacitor with a sacrificial silicon nitride sidewall, said method comprising the steps of: patterning a silicon substrate to form a trench region in said substrate; forming a TEOS oxide layer on said silicon substrate and filling in said trench region; etching said TEOS oxide layer to a first level, wherein a portion of said TEOS oxide layer is remained in said trench region, thereby a portion of a trench sidewall of said trench region is exposed; forming a thermal collar oxide on said trench sidewall; forming a silicon nitride sidewall on said collar oxide; removing said portion of said TEOS oxide layer to expose a bottom portion of said trench region; doping ions over said bottom portion of said trench region to form a bottom cell plate; etching a portion of said bottom cell plate to form a fresh trench region, wherein said process is done by using said silicon nitride sidewall as a barrier; removing said silicon nitride sidewall; forming a dielectric film along a surface of said bottom cell plate, said collar oxide, and said silicon substrate; forming a first conductive layer on said dielectric film and refilling in said trench region; etching said first conductive layer to a second level to form a storage node of said trench capacitor and exposing a portion of said collar oxide; etching said exposed dielectric film and portion of said collar oxide; forming a second conductive layer on said first conductive layer; and etching back said second conductive layer to a third level to form a buried strap in said trench region.
 16. The method according to claim 15, wherein said step of etching a portion of said bottom cell plate is done by wet etching.
 17. The method according to claim 16, wherein said step of wet etching is done by HNO₃/CH₃COOH/HF/DI (deionized) H₂O solution.
 18. The method according to claim 15, wherein said doping ions is selected from the group of arsenic and phosphoric ions.
 19. The method according to claim 15, wherein said silicon nitride sidewall has a width about 10 to 12 nm.
 20. The method according to claim 15, wherein said third level is below a top surface of said silicon substrate. 